Method of increasing the durability of nozzles for arc-plasma-torches with high power density and a connection arrangement for carrying out the process



GR 313541289 s 0 1967 L. HAASE ETAL 3,35 89 METHOD OF INCREASING THE DURABILITY OF NOZZLES FOR ARC-PLASMA-TORCHES WITH HIGH POWER DENSITY AND A CONNECTION ARRANGEMENT FOR CARRYING OUT THE PROCESS Filed Aug. 15, 1966 4 6 INVENTORS LOTHAR HAASE B RUDOLF POCHERT ATTORNEY.

United States Patent METHOD OF INCREASING THE DURABILITY OF NOZZLES FOR ARC-PLASMA-TORCHES WITH HIGH POWER DENSITY AND A CONNECTION ARRANGEMENT FOR CARRYING OUT THE PROCESS Lothar Haase and Rudolf Pochert, Dresden, Germany, assiguors t0 Kjellberg Elektroden & Maschineu G.m.b.H. in Verwaltung, Finsterwalde, Germany, a corporation of Germany Filed Aug. 15, 1966, Ser. No. 572,379

6 Claims. (Cl. 219-131) ABSTRACT OF THE DISCLOSURE A method of increasing the durability of nozzles for plasma arc torches with high power density, which comprises the steps of starting a main arc above a work piece with a lower output than for initial cutting, efiecting the changing over to the required output for initial cutting and starting from above said work piece, respectively, automatically by means of a plasma burner circuit cornprising a main arc circuit and a pilot arc circuit, producing a pilot are by means of a plurality of first circuit elements including a pilot arc source, a cathode, a nozzle of said plasma arc torches and a current relay, the first circuit elements being disposed in series, producing the main are from the pilot arc by means of a plurality of second circuit elements including a main arc source, the cathode, the nozzle of the plasma arc torches and the work piece, the second circuit elements being disposed in series, controlling the power of the main are by means of a control circuit, disposed in parallel with the main arc circuit and including a plurality of third circuit elements comprising a control current source, a variable resistor, and a dropping resistor, the third circuit elements being disposed in series, and bridging said dropping resistor by means of a make contact of said current relay in said pilot arc circuit.

The present invention relates to a method of increasing the durability of the nozzle life of arc plasma torches with high power density and a connection system for carrying out the process.

Arc plasma torches are known and operate on the principle of an ionized gas are ensuring high temperatures and outputs for the working of materials, preferably metals. Most frequently are plasma torches are used for cutting oif work pieces.

As concerns the electrical connection system for operation of an arc plasma burner, the so-called nozzle and work polarity method is known. By a nozzle polarity method, a method of operation is understood, Where the electric arc strikes between the cathode and the nozzle, i.e. the cathode is connected with the negative pole of the power source and the nozzle is connected with the positive pole. With the work polarity method, the work to be treated is connected with the positive pole. However, this is possible only with electrically conductive materials. It is known, furthermore, that for ignition of an arc plasma torch, first an auxiliary are, called also ignition or pilot arc, is started, which burns with low output between the nozzle and the cathode and, on approach to the work piece, is converted into the main arc, which then burns with high output between the cathode and the work piece.

During the initial cutting or cutting-in operation above the Work piece, the nozzle is subjected to the highest load. Frequent'y flash-overs (double arc, i.e. part of the output flows over the nozzle) between the cathode and the nozzle occur during this operation, which destroy the nozzle. In

order to avoid this drawback, a method is known, where the main are for the initial cutting or cutting-in operation is initiated with reduced output, while the full output is switched on only upon arrival of the additional gas.

Experiences gathered during the initial cutting operation have shown that a definite minimum initial-cutting output must be present to ensure reliable initial cutting.

If the work piece is to be cut in now, the initial cutting output stated above is still too high, so that again fiashover between the cathode and the nozzle is possible, whereby the nozzle is damaged, since, at the moment of ignition of the pilot arc, the arc plasma burner is already positioned above the work piece, and the main arc is started immediately afterwards.

It is, therefore, one object of the present invention to provide a method of increasing the durability of nozzles for arc-plasma-torches with high power density and a connection arrangement for carrying out the process, which increases the durability of the nozzle life of arc plasma torches, and avoids the disadvantages resulting from the known methods.

It is another object of the present invention to provide a method of increasing the durability of nozzles for arcplasma-torches with high power density and a connection arrangement for carrying out the process, which ensures both reliable initial cutting and cutting in without damaging the nozzle.

It is still another object of the present invention to provide a method of increasing the durability of nozzles for arc-plasma-torches with high power density and a connection arrangement for carrying out the process, wherein the main arc is started for cutting in (above a work piece) with a lower output than for initial cutting, and the changing-over to the respectively required output for initial cutting or cutting in is effected by hand or automatically.

It is yet another object of the present invention to provide a method of increasing the durability of nozzles for arc-plasma-torches with high power density and a con nection arrangement for carrying out the process, wherein the connection system for carrying out the process comprises a resistor which can be bridged over and which is included in the main circuit or in the control circuit of the power source of the plasma torch.

It is also an object of the present invention to provide a method of increasing the durability of nozzles for arc plasma-torches with high power density and a connection arrangement for carrying out the process, wherein it is particularly advantageous to bridge the resistor by a make contact of a current relay, which is included in the circuit of the pilot arc.

It is a further object of the present invention to provide a method of increasing the durability of nozzles for arcplasma-torches with high power density and a connection arrangement for carrying out the process, wherein it is possible that the resistor is bridged over by a make contact of a voltage relay, which is connected in parallel with the dropping resistor of the pilot arc.

The technical and economical advantages, especially the technical progress of the present invention resides in the fact, that the nozzle life is no longer reduced now by electric flash-overs, but only by occasional carelessness of the operator, such as by touching the nozzle when removing a cut-off work piece, whereby a short circuit is caused.

With these and other objects in view, which will become apparent in the following detailed description, the present invention will be clearly understood in connection with the accompanying drawing, in which:

FIGURE 1 is a diagram of a connection system comprising the main circuit and the pilot circuit, and a current relay included in the pilot circuit;

FIG. 2 is a diagram of a connection system of the main and pilot circuits including a voltage relay joined up in the pilot circuit; and

FIG. 3 is another embodiment of a diagram of the connection system according to the present invention, disclosing the pilot arc and the main arc during an initial cutting operation.

The initial cutting operation of an arc-plasma-torch takes place in about the following order:

Referring now to the drawings, FIGS. 1-3 disclose wiring diagrams of work-polarized arc-plasma-torches cutting plants. It is a common practice to ignite, first of all, an auxiliary are or so-called pilot arc. This pilot arc has a low output supplied, according to FIG. 1, by a separate pilot power source 5 or, according to FIGS. 2 and 3, by the main arc power source 9 by means of a dropping resistor 14, and it burns between the cathode 1 and the nozzle 3. The pilot arc, originating from the nozzle 3 establishes an electric connection as soon as it touches the work piece and, thereby, initiates or starts the main arc.

The cathode 1 of the plasma torch is connected with the negative pole of the power source 9 and the work piece 2 with the positive pole of the power source 9.

Consequently the main arc burns between the cathode 1 and the work piece 2. Upon ignition by the plasma flame of the pilot arc, the main arc will flow the way of the least electric resistance. This way it moves through the nozzle 3. The working gas of the plasma torch, for example argon, which supplies the discharge plasma in the beginning, has a higher electric resistance than the route: cathode 1-nozzle 3-work piece 2. The additional gas, preferably hydrogen, entering the discharge space on starting of the main arc, together with the working gas forms the discharge plasma which, on constriction through the nozzle 3, is sup plied to the work piece 2.

In either of the supply lines for the working gas and the additional gas a solenoid valve is arranged, which releases the gas flow into the discharge space. When starting an ignition, first of all the solenoid valve for the working gas is actuated and the pilot arc lighted. An electrically conductive plasma stream 4 is formed, as shown in FIG. 3. If now, for example, in an initial cutting operation, the plasma torch is approached to the work piece 2, the main arc is developed, which produces the plasma stream 4'. The formation of the main arc is connected with a rapid current increase in the main circuit. This current increase releases the solenoid valve for admission of the additional as. g During the period between ignition of the main arc and arrival of the additional gas, the main are, consequently, burns by means of nozzle 3, which consists of copper, and destroys it. In order that the nozzle 3 is not destroyed by this current increase, the current must be limited in the main power source, until also the additional gas takes part in the discharge.

Hydrogen used as additional gas gives a plasma with high energy content and high electric resistance, whereby the current intensity of the plasma stream 4' is again reduced. A method for realization of a starting method of the main are like this is now described:

The increase of the durability of nozzles for arc-plasmatorches with high power density is achieved such that an auxiliary arc is ignited first, whereupon the main arc is started with reduced power and switched to full power only later, preferably at about the moment of arrival of additional gas in the discharge space. As a result thereof, the nozzle is no longer subject to overload in the starting phase, so that damages are avoided. The gas-control device for the additional gas (e.g. a solenoid valve) is advantageously arranged near to or in the torch. In case of rectifier operation the transition from reduced to full power may be brought about by converters or generators which are placed in the main circuit. Their power is regulated by control of the exciter circuit. This method may be applied with similarly good results, as well for work-poled, as also for nozzle-poled plasma torches.

In the known connection of a Work-poled arc-plasmatorch the main arc gets at once the full load (after the section cathode-anode has been ionized by the auxiliary are) for example, by approach of the torch to the work piece. The main arc current flows from the negative pole of a direct-current source, such as a converter, through a current relay and the cathode, through the constriction nozzle to the Work piece (anode) and returns to the positive pole of the current source. The additional gas, e.g. hydrogen, which is switched in by a current relay arrives in the discharge space with a time lag as compared to the setting-in of the main arc. This time lag is conditioned by the gas in the hose line and by the relay time delay. Sparks fiow over between the nozzle and work piece (double arcs; arc burning partly or completely between cathode and nozzle and further between nozzle and work piece) occurs especially in torches of higher power due to the immediate setting-in of the full power and the delayed arrival of the additional gas. This leads to damage or break-down of the nozzle, especially when the torch must stand above the work piece already during ignition of the auxiliary arc, as for example, for cutting out of shaped parts.

According to this arrangement the direct current source is choked such, that before the arrival of the additional gas (starting phase) the main arc receives only so much power that the main arc is just maintained, While flow overs cannot yet occur. This can be achieved in a manner, that by example, in case of converter or generator operation, a series resistance is preferably placed in the exciter circuit or that in case of rectifier operation the power output is reduced by other suitable means.

After the main arc has been ignited in a known manner with its power reduced by the above-described measures, the admission of the additional gas, e.g. hydrogen, as well as the transition to full power is started preferably by a relay which is excited by the main arc. The full power is reached preferably by means of a contactor bridging the mentioned series resistance.

Since with converters or generators the build-up of the exciting field is dependent on time, the transition from the reduced to the full power is not sudden, but proportional to the increase in the intensity of the exciting field. In operation of plasma torches, especially with non-regulatable rectifiers, reduction of the power up to the time of arrival of the additional gas in the discharge space can be brought about for example, by a time-relay element or by one or several converters which are joined up in the main circuit and controlled in the described manner.

To reduce the time up to the arrival of the additional gas, the gas-control device for the additional gas, e.g. solenoid valve, is attached near the torch or directly at the torch or is installed in the torch.

It is advantageous, if the auxiliary arc can be switched off. The switching off may be controlled for example, by a current relay. The nozzle is thereby rendered completely without current, which contributes further to an increase in the durability of the nozzle.

The process and the corresponding arrangement can be applied with the same good result also to nozzle-poled plasma torches. The starting process, which is made possible by the described arrangement does not require the operator to perform any more switching operations in addition to those which were so far necessary and can take place within tenths of seconds.

When this arrangement is applied with high are powers of the magnitude of kw., no visible signs of Wear are found at the nozzle after several hundred cutting operations and also the cathode is well preserved. This is true also in such cases when the torch has to stand above or at the workpiece already during ignition of the auxiliary arc and when the main are sets in at once, as for example in case of cutting out of shaped parts. The diameter of the necessary starting hole in the workpiece has to be equal only to the width of the cutting gap, which for example in case of a material thickness of 100 mm. and an are power of 100 kw. is equal to about mm. The arrangement and the process render it possible to use are plasma torches with a high power area ratio with an economical service life of the nozzle. This is especially advantageous also in so far as are plasma torches of high power have a higher efficiency than such of low power.

Cutting in now involves the disadvantages, that the current increase in the main circuit takes place more quickly than with the initial cutting operation, as the main arc in the initial cutting operation develops along the work edge and is dependent upon its length (see FIG. 3), while with the cutting-in operation the main arc develops on the surface of the work piece, whereby sufficient electric contact with the work piece is immediately ensured. The present invention now assures, that the'output of the main arc for cutting in is more reduced than for initial cutting. For change-over of the output for the initial-cutting or cutting-in operation, respectively, according to the present invention, the pilot arc current is used as a criterion, as the pilot arc burns longer with the initial cutting operation, than with the cutting-in operation. In the pilot arc circuit of FIG. 1, shown in dotted lines, which is fed by the pilot arc power source 5, a current relay 6 with delayed attraction is included. By means of a make contact 7 the current relay 6 is capable of short-circuiting a dropping resistor 8 in the control circuit of the main arc power source 9, provided it is passed for some time by the pilot arc current and attracts its armature. The control circuit of the main are power source 9 is fed by a control current source 10. By the variable resistor 11 in the control circuit of the main arc power source 9, the output of the main arc is determined. According to the position of the resistor 11, the control current, such as the excitation current of converters or the bias current of transductors, is either reduced or increased.

FIG. 1 shows how the dropping resistor 8 is joined up in the control circuit of a transductor 12.

Below the further mechanism of operation, according to the present invention will be described:

For an initial cutting operation the plasma torch is positioned off the work piece 2 (FIG. 3). Upon ignition of the pilot arc the plasma torch is moved towards the work piece. This may last several seconds, so that the pilot arc and, with it, the plasma stream are fully developed. Meanwhile also the delay-action current relay 6, which is passed by the pilot current, has attracted and short-circuits with its make contact 7 the dropping resistor 8. Hereby the current in the control circuit of the main arc power source is increased and, consequently, also the output of the main arc for the initial cutting operation. After the plasma stream 4 has touched the work piece 2, the main arc is started, and the pilot arc goes out or is switched off. Now the make contact 7 is opened again and the plasma torch is ready for another initial cutting operation.

For a cutting-in operation the plasma torch, as already explained, is positioned above the work piece 2 (FIGS. 1 and 2). Consequently the pilot arc is immediately converted into the main arc. This happens in a very short time, so that the current relay 6, owing to its delayaction does not attract, whereby the dropping resistor 8 in the control circuit of the main are power source 9 remains effective. This means a lower initial output of the main arc for cutting in compared with the initial cutting operation.

As shown in FIG. 2 in the majority of the plasma fusion cutting plants a resistor 14 is used for limiting the pilot arc current. According to the present invention, instead of current relay 6 a normal voltage relay 13 with parallel-connected capacitor 16 and series resistor is preferably connected in parallel with resistor 14 to ensure the required delay of attraction; this voltage relay replaces the function of the current relay 6.

In a further embodiment according to FIG. 3, the variable resistor 11 and the dropping resistor 8 are included in the main arc circuit, and the latter is short-circuited for the initial cutting operation by the make con- 5 tact 7 of the current relay 6 joined up in the pilot circuit or of a voltage relay 13, as shown in FIG. 2. The resistors 11 and 8 and the make contact 7, however, have to be amply dimensioned for this purpose, as they must carry the whole current of the main arc.

While we have disclosed several embodiments of the present invention, it is to be understood that these embodiments are given by example only and not in a limiting sense, the scope of the present invention being determined by the objects and the claims.

We claim:

1. A connecting system for increasing the durability of nozzles for are plasma torches with high power density, comprising:

a main circuit of the plasma burner including a main arc power source, a cathode, a work piece and a dropping resistor, disposed in series therein,

means for bridging over said dropping resistor,

a pilot arc circuit,

said means for bridging over said dropping resistor comprising a make contact, and

a delay-action current relay disposed in said pilot arc circuit and operatively connected with said make contact.

2. The connection system, as set forth in claim 1, which includes a pilot arc circuit,

said means for bridging over said dropping resistor comprising a make contact, and

a voltage relay and a series resistor disposed in parallel in said pilot arc circuit.

3. A method of increasing the durability of nozzles for plasma arc torches with high power density, comprising the steps of starting a main are above a work piece with a lower output than for initial cutting,

elfecting the changing over to the required output for initial cutting and starting from above said work piece, respectively, automatically by means of a plasma burner circuit comprising a main arc circuit and a pilot arc circuit,

producing a pilot arc by means of a plurality of first circuit elements including a pilot arc source, a cathode, a nozzle of said plasma arc torches and a current relay, said first circuit elements being disposed in series,

producing said main arc from said pilot are by means of a plurality of second circuit elements including a main arc source, said cathode, said nozzle of said plasma arc torches and said work piece, said second circuit elements being disposed in series,

controlling the power of said main arc by means of a control circuit, disposed in parallel with said main arc circuit and including a plurality of third circuit elements comprising a control current source, a variable resistor, and a dropping resistor, said third circuit elements being disposed in series, and

bridging said dropping resistor by means of a make contact of said current relay in said pilot arc circuit.

4. A method of increasing the durability of nozzles for 65 plasma arc torches with high power density, comprising the steps of starting a main are above a work piece with a lower output than for initial cutting,

effecting the changing over to the required output for initial cutting and starting from above said Work piece, respectively, automatically by means of a plasma burner circuit comprising a main arc circuit and a pilot arc circuit,

a current relay disposed in said pilot arc circuit limiting the pilot arc current by means of a resistance, a

voltage relay with a condenser disposed parallel thereto, and a dropping resistor,

producing said main arc from said pilot are by means of a plurality of second circuit elements including a main arc source, a cathode, said nozzle of said plasma arc torches and said Work piece, said second circuit elements being disposed in series,

controlling the power of said main are by means of a control circuit, disposed in parallel with said main arc circuit and including a plurality of third circuit elements, comprising a control current source, a variable resistor, and a dropping resistor, said third circuit elements being disposed in series, and

bridging said dropping resistor by means of a make contact of said current relay in said pilot arc circut.

5. A method of increasing the durability of nozzles for plasma arc torches with high power density, comprising the steps of starting a main are above a work piece with a output than for initial cutting,

effecting the changing over to the required output for initial cutting and starting from above said work piece, respectively, automatically by means of a plasma burner circuit comprising a main arc circuit and a pilot arc circuit,

a current relay disposed in said pilot arc circuit, and

lower short circuiting a control circuit during initial cutting by the working cycle of said current relay.

6. A method of increasing the durability of nozzles for plasma arc torches with high power density, comprising the steps of starting a main are above a work piece with a lower output than for initial cutting,

effecting the changing over to the required output for initial cutting and starting from above said work piece, respectively, automatically by means of a plasma burner circuit comprising a main arc circuit and a pilot arc circuit, and

limiting the pilot arc circuit by means of a resistance,

a voltage relay with a condenser disposed parallel thereto, and a dropping resistor.

References Cited UNITED STATES PATENTS 1,346,164 7/1920 Bennett 219- 2,315,625 4/1943 King 219131 2,922,871 1/1960 Hachman et al. 219-131 2,993,984 7/ 1961 Sullivan 219-131 RICHARD M. WOOD, Primary Examiner. 

1. A CONNECTING SYSTEM FOR INCREASING THE DURABILITY OF NOZZLES FOR ARC PLASMA TORCHES WITH HIGH POWER DENSITY, COMPRISING: A MAIN CIRCUIT OF THE PLASMA BURNER INCLUDING A MAIN ARC POWER SOURCE, A CATHODE, A WORK PIECE AND A DROPPING RESISTOR, DISPOSED IN SERIES THEREIN, MEANS FOR BRIDGING OVER SAID DROPPING RESISTOR, A PILOT ARC CIRCUIT, SAID MEANS FOR BRIDGING OVER SAID DROPPING RESISTOR COMPRISING A MAKE CONTACT, AND A DELAY-ACTION CURRENT RELAY DISPOSED IN SAID PILOT ARC CIRCUIT AND OPERATIVELY CONNECTED WITH SAID MAKE CONTACT. 